Electromagnetic devices, or electric machines, of the reluctance type (so-called “switched or synchronous reluctance machines”), are known which have anisotropic rotor structures, consisting of alternating portions of magnetic and non-magnetic material.
The portions of non-magnetic material constitute electromagnetic flux barriers and may also consist of voids (in this case, air constituting the non-magnetic material), of suitable shape and dimension within rotor structures; the magnetic portions, having high magnetic permeability, constitute the magnetic poles of the rotor structure.
Examples of reluctance electric machines are described in document U.S. Pat. No. 5,818,140, which discusses some general rules for the design of an electric machine with reduced torque ripple, in particular as regards the number and the arrangement of the flux barriers in the rotor structure, and in documents WO 2012/000561 A1, WO 2012/000544 A1, WO 2011/154045 A1, U.S. Pat. Nos. 6,239,526 and 6,769,167, describing possible alternative embodiments, based in each case on the structure described in general in the abovementioned document U.S. Pat. No. 5,818,140.
By way of example, FIG. 1 shows a section of a synchronous reluctance electric machine, of a known type, in particular made according to the teachings contained in document U.S. Pat. No. 5,818,140, and indicated as a whole by 1.
The electric machine 1 comprises a rotor structure 2, having a radial symmetry about a longitudinal axis of extension (typically, the rotor structure 2 consists of a plurality of discs, one of which is shown in FIG. 1, stacked along the longitudinal axis L); and a stator structure 3, arranged in a radially external position with respect to the rotor structure 2, and magnetically coupled to the same rotor structure 2.
The stator structure 3 comprises a plurality of pole expansions 4a (so-called “teeth”), around which corresponding coils (or windings) are wound, not shown in FIG. 1, designed to generate the magnetic field, and electrically connectable to an electrical supply source (not shown), when the electric machine 1 operates as an electric motor by transforming electrical energy into mechanical energy, or to electrical devices or loads, when the electric machine 1 operates as a generator transforming mechanical energy into electrical energy. Stator openings 4b, so-called “stator slots”, are defined between the pole expansions 4a. 
The rotor structure 2 is provided with a central opening 5 having a centre O, designed to be engaged by a rotation shaft (not shown here), and has an annular arrangement about the same central opening 5.
In particular, the rotor structure 2 has a plurality of magnetic portions 6, made of a suitable ferromagnetic material, and a plurality of flux barriers 8, interposed between and separating them from the magnetic portions 6.
The flux barriers 8 may for example be formed by removal of material, by means of cutting, for example by laser or localised thermal treatments, and include void regions; alternatively, the flux barriers 8 may be made of a suitable non-ferromagnetic material.
The flux barriers 8 are configured to generate anisotropy in the rotor structure 2, so as to define minimum reluctance paths (so-called “d axes”) and maximum reluctance paths (so-called “q axes”), for the poles of the same rotor structure 2 (four in number in the example shown in FIG. 1).
In particular, each pole comprises several (typically three to five) magnetic portions 6 and interposed flux barriers 8, having a corresponding shape. An axis of radial symmetry of each pole coincides with the maximum reluctance axis q.
Considering a pair of orthogonal d−q axes, so that the d axis coincides with a direction of minimum reluctance and the q axis coincides with a region of maximum reluctance of the rotor structure 2, the value Ld/Lq is defined as the “anisotropy ratio”, where LD and Lq indicate the inductance values in the two directions.
It is indeed this anisotropy within the rotor structure 2 which enables the electric machine 1 (operating as a motor or as a generator) to produce electromagnetic torque, whose value is greater, the higher the anisotropy ratio Ld/Lq.
In particular, when operating as an electric motor, the application of appropriate excitation currents to the coils of the stator structure 3 generates a movement of the rotor structure 2 to align the direction of maximum magnetic permeability (i.e. the d axis) with the direction of the resulting stator magnetic flux. This movement results in an overall rotation of the rotor structure 2 of the electric machine 1, about the axis of rotation. Similarly, during operation of the electric machine 1 as a generator, rotation of the rotor structure 2 causes a variation in the stator magnetic flux and the generation of a resulting electric current in the corresponding coils.
As described in detail in the above-mentioned document U.S. Pat. No. 5,818,140 (to which reference can be made for further details), in order to obtain the desired electromagnetic performance, the number of flux barriers 8 (which defines the “equivalent rotor slots”) is appropriately selected according to the number of pole expansions 4 of the rotor structure 3 (which defines the “stator slots”). In particular, real slots of the rotor structure 2, indicated by circles in FIG. 1, are defined at the ends of the flux barriers 8, spaced by a rotor pitch pr; so-called virtual slots of the same rotor structure 2, indicated by x, are defined on the circumference of the rotor structure 2, repeating, in an angularly uniform way about the central axis, the rotor pitch pr.
In particular, it is preferable that the total number of equivalent rotor slots, real and virtual, nr, for each pair of magnetic poles of the rotor structure 2, satisfies the following relationship with the number of stator slots ns: ns−nr=±4.
The arrangement of the flux barriers 8 inside the rotor structure 2 derives from the fulfillment of this relationship.
In particular, in the embodiment illustrated in FIG. 1, the flux barriers 8 have the shape of concentric semi-ellipses (in the section illustrated, transverse to the longitudinal axis L), symmetrical about the q axis and with increasing axis, from the outer surface to the inner surface of the rotor structure 2, with the major axis of each ellipse joining a pair of real rotor slots. Overall, the flux barriers 8 have a regular shape, convex with respect to the centre O of the central opening 5 of the rotor structure 2.
The rotor structure 2 further comprises a plurality of mechanical connection elements, hereinafter defined simply as “bridges”, designed to mutually couple radially adjacent magnetic portions 6, in order to produce an adequate mechanical resistance for the rotor structure 2, in particular in relation to the centrifugal force due to rotation.
In detail, there are circumferential bridges 10, at the ends of the flux barriers 8 and at the outer lateral surface of the rotor structure 2 (i.e. in the vicinity of the gap in the magnetic coupling between the stator structure 3 and the rotor structure 2); and furthermore radial bridges 12, which pass through the flux barriers 8.
In particular, as shown in FIG. 1, a radial bridge 12 can be provided for each flux barrier 8, or, as shown in the embodiment of FIG. 2 (also of a known type) two (or more) radial bridges 12 can be provided for each flux barrier 8, in order to increase a mechanical resistance.
The radial bridges 12 can also be used for housing the magnets inside the rotor structure 2.
DE 10 2014 201740 A1 describes a rotor structure for a reluctance machine, wherein the flux barriers are designed in a manner substantially similar to that described in U.S. Pat. No. 5,818,140 cited previously; the bridges that connect the magnetic portions are in this case arranged overall along a circumferential line.
US 2006/108888 A1 describes a further rotor structure, having only two poles, wherein the flux barriers of each pole are continuous and no provision is made for the presence of radial bridges connecting the magnetic portions.
The present Applicant has realized that, from the point of view of mechanical resistance, increasing the thickness of the circumferential bridges 10 and/or the radial bridges 12, results in an overall reduction in the maximum stress to which the rotor structure 2 is subjected.
However, increasing the thickness of the same circumferential bridges 10 and/or radial bridges 12 causes a reduction in the electromagnetic performance of the electric machine 1, in particular a reduction of the mechanical power produced for the same electrical power input to the electric machine (or, if the operation is generating, a reduction in the electric power generated, for the same mechanical power input).
The present Applicant has realized that, considering a desired electromagnetic performance, known solutions for synchronous reluctance electric machines do not allow to optimise mechanical resistance or strength properties.